When transmitting communication signals between a master station and a slave station, signals present in analog form must frequently be transmitted in digital form over transmission circuits connecting the master station with the slave station. These communication signals are frequently transmitted in pulse code modulated (PCM) form over the relevant transmission circuit. Before the communication signals are in this form, i.e., before they appear as PCM signals, they are frequently available in the pulse amplitude modulated (PAM) form initially. In PAM form, the PCM signals transmitted over the transmission circuit must be relayed at the receiver.
In connection with the conversion of PAM signals into PCM signals and of PCM signals into PAM signals, a commonly known technique (West German Unexamined Pat. application No. 2 024 963) is to carry out the conversion in accordance with the shape of a nonlinear bend characteristic, rather than carrying out a linear conversion. This conversion is accomplished at the transmitter in accordance with the shape of a companding (i.e. compressed and expanded) characteristic and at the receiver in accordance with the shape of an expansion characteristic. The PAM signals are fed to one input of a comparator, to the other input of which is fed the output voltage of a coder. The output of this coder varies in response to the coupling of a linearly varying input voltage in accordance with a logarithmically varying quantization characteristic. The control of the delivery of the output voltage from the coder takes place from a code control which, in addition, controls the counting operation of a counter. The counter position represents the code chain corresponding to the relevant instantaneous amplitude of the output voltage of the coder. In case of correspondence between the amplitude of the PAM signal and the output signal of the coder, the operation of the counter is stopped.
The counter position of the counter is then transmitted as the PCM signal corresponding to the aforesaid PAM signal. This PCM signal is introduced into a shift register at the receiver; this triggers a decoder over a code control. This decoder delivers an output signal in response to this triggering concerned, the output signal varying in accordance with the shape of an oppositely directed expansion characteristic matching the aforesaid companding characteristic. When an amplitude of the output voltage of the decoder corresponding to the code chain of the PCM signal is attained, the output voltage thereof is delivered at the receiver as a PAM signal corresponding to the PCM signal.
Although in the prior art circuit arrangment described hereinabove, PAM signals can be converted into PCM signals, account being taken of a companding characteristic, and PCM signals can be converted into PAM signals, account being taken of an oppositely directed expansion characteristic, the required circuit complexity is fairly great. This is particularly evident if one considers employing the arrangements referenced above in a communication system in which each of the individual message centers has a master station and a slave station. In this case, both prior art arrangements mentioned hereinabove must be provided for each such message center.
It is an object of the invention to provide a method of converting analog signals into PCM signals and PCM signals into analog signals with comparatively less circuit complexity than in prior art circuits, such as the one described above.